World Trade Center 7 Steel Samples Show Severe Damage Due To A High Temperature Corrosion Attack Which Melted The Steel

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http://www.springerlink.com/content/g5w603461r3078t3/


World Trade Center 7 Index 


NIST Claims To Have No Steel Samples From WTC7 




FEMA: World Trade Center Tower Steel Sample Shows Severe Damage Due To A High Temperature Corrosion Attack Which Melted The Steel


[Structural Steel Physical Properties Of Structural Steel] 
World Trade Center 7 Demolition 
NIST Claims Molten Steel In The Wreckage Was Due To Long Term Combustion Within The Debris Pile 


Several samples from Buildings 7, and 1 or 2 of the World Trade Center were collected during the FEMA forensic investigation shortly after September 11, 2001. Macroscopically these samples revealed severe “erosion” of the steel beam thickness from 1/2 inch to total metal loss in some regions, as well as localized plastic deformation. We were asked to determine the cause of this “erosion” as well as the temperature that the steel experienced. wpi.edu


The as-fabricated microstructure consisted of a hot worked banded structure of ferrite and pearlite. In severely “eroded” regions where the thickness had been reduced to less than a 1/16 of and inch significant decarburation was observed. In addition, some pearlite bands presented regions that had re-austentized as well as regions where the pearlite had started to spheroidize. These observations indicate that steel had experienced temperature between 550 and 850°C. wpi.edu


An examination of the “slag” that formed on the surface of the steel found iron oxides and iron sulfides. It appeared that the “slag” was liquid at high temperature and easily attacked the grain boundaries. A eutectic microstructure was seen within the “slag” of iron oxides and iron sulfides. If these compounds were pure Wustite (FeO) and Iron sulfide (FeS), the eutectic temperature is 940°C. It appears that the severe “erosion” was due to the sulfidation and oxidation (i.e. hot corrosion) of the steel followed by the liquid “slag” attack of the grain boundaries. wpi.edu


"A combination of an uncontrolled fire and the structural damage might have been able to bring the building down, some engineers said. But that would not explain steel members in the debris pile that appear to have been partly evaporated in extraordinarily high temperatures" nytimes.com nytimes.com


FEMA_figure_C-1_Eroded_A36_wide-flange_beam.jpg 
fema.gov 


The results of the examination are striking. They reveal a phenomenon never before observed in building fires: eutectic reactions, which caused intergranular melting capable of turning a solid steel girder into Swiss cheese. The New York Times described this as "perhaps the deepest mystery uncovered in the investigation." prisonplanet.com


"A one-inch column has been reduced to half-inch thickness. Its edges - which are curled like a paper scroll - have been thinned to almost razor sharpness. Gaping holes - some larger than a silver dollar - let light shine through a formerly solid steel flange. This Swiss cheese appearance shocked all of the fire-wise professors, who expected to see distortion and bending- but not holes." wpi.edu


fig1.gif 

Severely eroded I beam cross sections, nominal composition (%) of A36 steel plate is: (0.29C max, 0.80–1.2Mn, 0.04P, 0.05S, 0.15–0.3Si bal Fe). tms.org


fig3.gif 

Oxidation and intergranular melting; unetched. tms.org


fig5.gif 

Eutectic formation (iron oxide-iron sulfide), etched 4% natal. tms.org


fig4.gif 

An EDX Analysis of eutectic region. tms.org


fig2.gif  fig2b.gif 

The microstructure of unaffected A36 steel: (a-left) white-ferrite, dark-pearlite and (b-right) pearlite region. Pearlite forms in bands due to manganese segregation and prior hot working. tms.org



C.2 Sample 1 (From WTC 7)

      Several regions in the section of the beam shown in Figures C-1 and C-2 were examined to determine microstructural changes that occurred in the A36 structural steel as a result of the events of September 11, 2001, and the subsequent fires. Although the exact location of this beam in the building was not known, the severe erosion found in several beams warranted further consideration. In this preliminary study, optical and scanning electron metallography techniques were used to examine the most severely eroded regions as exemplified in the metallurgical

mount shown in Figure C-3. Evidence of a severe high temperature corrosion attack on the steel, including oxidation and sulfidation with subsequent intergranular melting, was readily visible in the near-surface microstructure. A liquid eutectic mixture containing primarily iron, oxygen, and sulfur formed during this hot corrosion attack on the steel. This sulfur-rich liquid penetrated preferentially down grain boundaries of the steel, severely weakening the beam and making it susceptible to erosion. The eutectic temperature for this mixture strongly suggests that the temperatures in this region of the steel beam approached 1,000 °C (1,800 °F), which is substantially lower than would be expected for melting this steel.

fema.gov Appendix C Page 1 




"Evidence of a severe high temperature corrosion attack on the steel, including oxidation and sulfidation with subsequent intergranular melting, was readily visible in the near-surface microstructure.”... “The thinning of the steel occurred by high temperature corrosion due to a combination of oxidation and sulfidation.”...“The unusual thinning of the member is most likely due to an attack of the steel by grain boundary penetration of sulfur forming sulfides that contain both iron and copper.”...“A liquid eutectic mixture containing primarily iron, oxygen, and sulfur formed during this hot corrosion attack on the steel.”... “The severe corrosion and subsequent erosion of Samples 1 and 2 are a very unusual event. No clear explanation for the source of the sulfur has been identified."

fema.gov Appendix C Page   


C.3 Summary for Sample 1

a. The thinning of the steel occurred by high temperature corrosion due to a combination of oxidation and sulfidation.

b. Heating of the steel into a hot corrosive environment approaching 1,000°C (1,800°F) results in the formation of a eutectic mixture of iron, oxygen, and sulfur that liquefied the steel.

c. The sulfidation attack of steel grain boundaries accelerated the corrosion and erosion of the steel.

fema.gov_C.3 


C.6 Suggestions for Future Research

The severe corrosion and subsequent erosion of Samples 1 and 2 are a very unusual event. No clear explanation for the source of the sulfur has been identified. The rate of corrosion is also unknown. It is possible that this is the result of long-term heating in the ground following the collapse of the buildings. It is also possible that the phenomenon started prior to collapse and accelerated the weakening of the steel structure. A detailed study into the mechanisms of this phenomenon is needed to determine what risk, if any, is presented to existing steel structures exposed to severe and long-burning fires.

fema.gov 


Appendix C

Figure C-5 Another hot corrosion region near the steel surface (etched with 4 percent nital).... C-3

Figure C-10 Grain boundary corrosion attack.... C-6

Figure C-11 Microstructure attack of a typical region showing the surface and grain boundary corrosion attack of Sample 2.... C-7

Figure C-12 Higher magnification of the region shown in Figure C-10.... C-7

fema.gov 


Chapter 8 Appendix C

8.2.8: Limited Metallurgical Examination

Two structural steel samples from the WTC site were observed to have unusual erosion patterns. One sample is believed to be from WTC 7 and the other from either WTC 1 or WTC 2.

8.2.8.1 Observations and Findings

a. The thinning of the steel occurred by high temperature corrosion due to a combination of oxidation and sulfidation.

b. Heating of the steel into a hot corrosive environment approaching 1,000°C (1,800°F) results in the formation of a eutectic mixture of iron, oxygen, and sulfur that liquefied the steel.

c. The sulfidation attack of steel grain boundaries accelerated the corrosion and erosion of the steel.

d. The high concentration of sulfides in the grain boundaries of the corroded regions of the steel occurred due to copper diffusing from the high-strength low-alloy (HSLA) steel combining with iron and sulfur, making both discrete and continuous sulfides in the steel grain boundaries.

8.2.8.2 Recommendations

The severe corrosion and subsequent erosion of Samples 1 and 2 constitute an unusual event. No clear explanation for the source of the sulfur has been identified. The rate of corrosion is also unknown. It is possible that this was the result of long-term heating in the ground following the collapse of the buildings. It is also possible that the phenomenon started prior to collapse and accelerated the weakening of the steel structure. A detailed study into the mechanisms of this phenomenon is needed to determine what risk, if any, is presented to existing steel structures exposed to severe and long-burning fires.

fema.gov 


Additional WTC7 Steel Information 


http://www.tms.org/pubs/journals/JOM/0112/Biederman/Biederman-0112.html


BBC "The Third Tower" At 48:00

[Professor Jonathan Barnett, Fire Protection Engineer, Simpson Gumpertz & Heger]

"It came from a much larger beam… This was the size of steel that they used in the construction of Tower 7. They didn't use this particular kind of steel in Towers 1 or Towers 2. So that's why we know its pedigree. It was a surprise to me because it was so eroded and deformed and so we took it for analysis in the lab."

http://video.google.com/videoplay?docid=9072062020229593250#

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